1. Field of the Invention
The present invention relates to an optical pick-up of an optical device or an optical disk device, and more particularly to a wave front aberration correcting mirror.
2. Description of the Invention
An information recording medium using an optical disk includes a compact disk (CD) and a digital video disk (DVD). In recent years, there has generally been employed a structure in which a plurality of recording media is read and written by the same optical disk device, and a technique for manufacturing the optical disk device to have a smaller size than that of a conventional device has been required. In particular, an optical disk device for a notebook PC has been increasingly required to reduce a size and a thickness. According to the development of a multimedia technology, moreover, a storage in an optical disk or a demand for a recording capacity tends to be increased year after year, and a recording density is enhanced by such means that (1) a blue laser having a shorter wavelength than that in a conventional art is used or (2) a numerical aperture (NA) of an objective lens is increased. In addition, (3) a recording area is increased by providing a plurality of recording layers in media so that a capacity can be increased.
The optical disk device is provided with a laser beam source, an optical pick-up and a photo-detector. A laser beam emitted from the laser beam source is collected onto the data plane of an optical disk through the optical pickup and is reflected, and is then received by the photo-detector, and information recorded in the optical disk is read or information is written to the optical disk. In this case, the wave front of the beam receives an aberration by various optical components or optical disks. For this reason, an aberration correction is indispensable for correctly reading and writing information. Referring to a dynamic aberration generated during the rotation of the optical disk or the different read of various layers, particularly, fixed correcting means formed by a lens or a diffracting optical unit constituting the optical pick-up is improper and a dynamic correction to be carried out by an actuator is indispensable.
The conventionally proposed aberration correcting means will be schematically described below with reference to the prior document.
In a method described in (JP-A-10-241201 Publication), a spherical aberration is corrected by moving a correcting lens by means of an actuator. However, this method is unstable for an optical pick-up in which an actuator portion is large, an excessive lens is required and a demand for a reduction in a size such as PC uses is great.
In a method described in (JP-A-10-134400 Publication), there has been disclosed a method of correcting a spherical aberration by moving one of collimator lenses through an actuator. Similarly, there is a problem in that the method is unstable for an optical pick-up in which an actuator portion is large and an excessive lens is required, and a demand for a reduction in a size such as PC uses is great.
An aberration correcting mirror described in (JP-A-10-039122 Publication) has such a structure as to bond a flange using a soft material to a mirror having a spherical initial shape and to stick a piezoelectric device to the back face of the flange, and serves to change the curvature of the mirror by the deformation of the piezoelectric device. In this method, however, there is a problem in that it is hard to inexpensively fabricate a small mirror with high precision and the mirror is deformed when bonding the flange or the piezoelectric device. Even if the bonding is carried out with high precision, moreover, there is also a problem in that the amount of the deformation of the piezoelectric device is remarkably small and the amount of the deformation which is required for an aberration correction cannot be obtained with an electrode to be a solid electrode which has been proposed and the structure of the piezoelectric device having surroundings fixed completely, and within the range of the practical magnitude of a voltage. Even if the deformation is carried out, furthermore, there is a problem in that it is hard to have an optional spherical shape through the deformation. The practical magnitude of the voltage indicates a smaller voltage than the upper limit of a voltage based on an insulating property or a polarization efficiency. In addition, the piezoelectric device of a bulk is used. For this reason, there is a problem in that a comparatively high voltage, for example, approximately 50 V is required for a driving voltage.
In a method described in (JP-A-2001-34993 Publication), there has been disclosed a method of applying a voltage with a structure in which a pair of opposed sides is fixed and the other pair of sides is free, thereby deforming the piezoelectric device to decrease a coma in a rectangle piezoelectric device. In this method, however, the piezoelectric device basically takes the shape of a wedge. For this reason, there is a problem in that a spherical aberration cannot be corrected even if a plurality of electrodes is combined with each other.
In a method described in (JP-A-2002-279677 Publication), there has been described an example in which an electrode taking such a shape as to correspond to a coma is formed on a piezoelectric device and the piezoelectric device is deformed by applying a voltage, thereby relieving the coma. Also in this method, however, a displacement is very small actually and such a displacement as to meet the aberration correction cannot be obtained within a range of the practical magnitude of a voltage if the surroundings of the piezoelectric device are fixed. Particularly, there is a problem in that this method is not suitable for an optical pick-up drive using an objective lens having a high NA.
As described above, there has been proposed that the wave front aberration of a beam is corrected by a method of mechanically moving the position of a lens in a conventional example (JP-A-10-241201 Publication and JP-A-10-134400 Publication) or mechanically deforming a mirror (JP-A-10-039122 Publication, JP-A-2001-34993 Publication, and JP-A-2002-279677 Publication) In the former example, there is a problem in that a driving device for changing the position of the lens is large and a demand for reducing a size is not satisfied. On the other hand, in the latter example, there is a problem in that the demand for reducing a size is satisfied and the amount of a deformation is small because the piezoelectric device is used. In order to explain this problem, the basic action of the piezoelectric device will be described with reference to FIG. 18. A piezoelectric unit is an electromechanical energy converting unit, and generates a mechanical stress to cause an elastic deformation when an electric field is applied. When an elastic unit is bonded to the piezoelectric unit, a whole material is deformed corresponding to the physical properties (elasticity) of each material by the action of the piezoelectric unit. FIG. 18 is a perspective view showing the state of the deformation of a piezoelectric unit before and after applying a voltage. It is assumed that a piezoelectric unit 1 obtained before the application of an electric field is a rectangular parallelepiped and is arranged with respect to an orthogonal coordinate system as shown in FIG. 18. Moreover, it is assumed that the piezoelectric unit 1 is previously subjected to a polarizing process in a +z direction. At this time, it is assumed that the piezoelectric distortion constant of the piezoelectric unit 1 is represented by a matrix expressed in a so-called d format (Equation 1), for example.
                    d        =                              [                                                            0                                                  0                                                                      -                    0.1360                                                                                                0                                                  0                                                  0.1360                                                                              0                                                  0                                                  0.3370                                                                              0                                                  0.5                                                  0                                                                              0.5                                                  0                                                  0                                                                              0                                                  0                                                  0                                                      ]                    ×                      10                                          -                9                            ⁢                                                                            ⁢                      (                          C              /              N                        )                                              [                  Equation          ⁢                                          ⁢          1                ]            
At this time, a distortion S based on an electric field E is expressed in a tensor form:Si=dijEj Herein, an index takes i=1, 2, 3, 4, 5, 6 and j=1, 2, 3. As shown in FIG. 18, only E3 is non-zero when an electric field is set into the +z direction. Therefore, only S1, S2 and S3 are non-zero based on the (Equation 1). In addition, only S1 is negative and S2 and S3 are positive. In accordance with the custom of a marking method related to a piezoelectric device, it is apparent that S1=Sxx, S2=Syy, and S3=Szz are set and only Sxx has a negative sign, and a contraction is thus carried out in an x direction and an expansion is performed in y and z directions. Referring to a piezoelectric unit 2 in FIG. 18, there is schematically shown the form of a deformation obtained by the expansion and contraction after the application of an electric field.
FIG. 19 is a sectional view showing a unimolf type piezoelectric device in which a piezoelectric unit 3 and an elastic unit 4 are bonded to each other. In the same manner as described above, it is assumed that the piezoelectric unit 3 should be polarized in the +z direction. The elastic unit 4 is bonded to the lower part of the piezoelectric unit 3. A displacement on an end face at a left side in the drawings is completely constrained and the other end is caused to be free. FIGS. 19(a) and (b) show states obtained before and after the application of an electric field, respectively. When the electric field is applied in the +z direction, the piezoelectric unit 3 tries to be contracted in the x direction in the same manner as the above description. Since a left end is fixed, however, the elastic unit 4 is downward convexed upon receipt of a bending moment. As a result, the elastic unit 4 is warped up in the +z direction. A whole displacement is determined by the elastic constants of the piezoelectric unit and the elastic unit and the thickness of a film in addition to the piezoelectric distortion constant To the contrary, when the electric field is applied in a −z direction, the piezoelectric unit 3 tends to be extended in the x direction so that the elastic unit 4 receives a bending moment having a reverse polarity, and is thus convexed upward and is warped in the −z direction (not shown).
Next, description will be given to the case in which a displacement is constrained on both ends of a piezoelectric device. FIG. 20 is a sectional view showing a unimolf type piezoelectric device in which the piezoelectric unit 3 and the elastic unit 4 are bonded to each other. In the case in which both ends are completely fixed differently from the case in which only one end is fixed, a bending moment is generated with difficulty so that a displacement is remarkably reduced as shown in FIG. 20. An electric field strength to be applied to the piezoelectric unit has a practical upper limit due to a limit such as a dielectric breakdown. For this reason, a deformation rarely appears within a range of the electric field strength. Even if a very small displacement is obtained, the amount of an aberration to be corrected is larger than that in the conventional art and the amount of the displacement of the shape of a mirror required for the correction is several to several tens times as large as the wavelength of a light to be used in case of an optical system using an objective lens having a high NA and an optical system having a short wavelength. In the structure shown in FIG. 20, therefore, it is impossible to obtain such a great displacement. In order to correct a spherical aberration, it is necessary to first cause the shape of the mirror to be circular and to secondly deform the mirror to be spherical. In order to carry out the deformation into a spherical surface, a symmetry about the optical axis of the shape of the mirror is very important. Accordingly, it is necessary to completely fix the circumference of the mirror in order to dynamically hold a circular mirror axially symmetrically. From the above description, it is apparent that the displacement of a piezoelectric device mirror having a circumference fixed completely is remarkably small. In the method according to the conventional art, accordingly, it is hard to achieve the aberration correction by the piezoelectric device.